What are the two types of endoplasmic reticulum?

The two types of endoplasmic reticulum are smooth and rough. The rough endoplasmic reticulum (RER) is studded with ribosomes on its cytoplasmic surface, giving it a "rough" appearance under a microscope. This structure is primarily involved in the synthesis of proteins that are either secreted from the cell, incorporated into the cell's plasma membrane, or sent to an organelle, like the lysosome. The presence of ribosomes is what distinguishes the rough ER from the smooth endoplasmic reticulum.

On the other hand, the smooth endoplasmic reticulum (SER) lacks ribosomes and has a smooth appearance. It is involved in the synthesis of lipids, metabolism of carbohydrates, detoxification of drugs and poisons, and storage of calcium ions. The functions of the smooth ER are essential for various cellular processes, particularly in glandular tissues and liver cells.

In summary, the correct classification of endoplasmic reticulum into smooth and rough is based on their distinct structural characteristics and functional roles within the cell.